1. Field of the Invention
The present invention relates to a honeycomb structure for fluid filtration, and more particularly to a honeycomb structure adapted for microfiltration or ultrafiltration in pharmaceutical preparation, food and beverage processing and other applications and for gas separation on the basis of Knudsen gas diffusion or capillary condensation.
2. Description of the Prior Art
In U.S. Pat. No. 4,069,157 granted to Hoover et al. on Jan. 17, 1978, there is disclosed an ultrafiltration device which comprises an integral support of porous ceramic material having substantially uniformly spaced and axially parallel passageways formed therein and a permselective membrane coated onto the surface of the passageways. The ultrafiltration device is characterized in that the surface area (SA) of the passageways in the porous support is about from 300 to 2,000 square meters per cubic meter of support, the pores in the support have diameters of about from 2 to 20 microns and the decimal proportion of the volume of the pores to the total volume occupied by the material (PV) is about from 0.30 to 0.60, the decimal proportion of the volume of the support material exclusive of the passageways to the total volume of the support (SV) is about from 0.40 to 0.75, and the permeability factor (PF) of the support, defined as PF=PV.times.SV/SA, is greater than about 1.0.times.10.sup.-4 meter.
In the patent to Hoover et al., it is pointed out that the permeability factor (PF) of the support is important to provide adequate flow of filtrate through the membrane and the ceramic support material to be carried through the porous support structure to the exterior surface of the support for collection. The patent to Hoover et al., however, fails to disclose or suggest the fact that the filtration efficiency of the device is greatly influenced by flow resistance of the permselective membrane and porous support structure. In fact, the flow resistance is determined by other complicated factors such as the cross-sectional shape and distribution of the pores in the support, the thickness of the membrane and the like. In this connection, it is to be noted that the permselective membrane of the patent to Hoover et al. consists of large particles of about from 2 to 20 microns, intermediate size particles and small particles of about from 0.02 to 4.0 microns which are irregularly accumulated in the pores of the support element. Such irregular accumulation of the particles will be an obstacle to the microfiltration of fluid and cause an unwanted clog of the pores in the membrane. Moreover, the permselective membrane is poor in resistance to heat since it is formed on the surface of the support element passageways using dispersions of the particles in a suspending liquid. The permselective thin membrane on the surface of the support element passageways is also less durable since 80-85% of the pores in the support element are in the 5-10 micron range.